The present invention relates to a colour display tube which can be used for image display as well as datagraphic display.
More particularly the present invention relates to a display tube comprising a channel plate electron multiplier and a cathodoluminescent screen formed by dots of one phosphor surrounded by one or two rings of other phosphors. For convenience of description this screen will be referred to as a dot and ring display screen.
A display tube having such a display screen is disclosed in British Patent Specification 1446774 (corresponding to U.S. Pat. No. 4,023,063). In this known tube the electron multiplier comprises a plurality of apertured dynodes which are insulated from each other. The apertures have a re-entrant shape in that they have their minimum cross-sectional areas at the input and output surfaces of each dynode. An apertured focusing electrode is mounted on the output dynode and is insulated therefrom. The apertures in the focusing electrode diverge in the direction towards the dot and ring display screen. In operation a substantially constant potential difference, which provides an accelerating field, is maintained between the last dynode and the screen. A positive voltage Vf between the last dynode and the focusing electrode is variable and serves to draw out the electrons and shape them into a beam. By varying the voltage Vf the size and shape of the electron beam emerging from a channel can be changed. More specifically, a circular "solid" beam of a minimum diameter is formed when the voltage Vf is zero (OV). By making the voltage Vf more positive, the electron beam is formed into an annular (or ring like) cross section the diameter increasing to a maximum at a typical maximum voltage of 140V.
A modification of this known tube is disclosed in British Patent Specification 1452554 (corresponding to U.S. Pat. No. 4,034,254) which is a Patent of Addition to British Patent Specification 1446774. In Specification 1452554 two focusing electrodes are provided. The first one has divergent apertures, which are smaller than the apertures in the dynodes, and serve to shape the electron beam emerging from the channel plate electron multiplier proper. The second focusing electrode has re-entrant shaped apertures and has a variable focusing voltage applied to it.
Whilst both these known display tubes are able to produce colour displays there is still a desire to improve on the quality of the dots and rings in order to get better colour purity.